This invention relates to a glow plug adapted to preheat the inside of a cylinder of a diesel engine and a method of manufacturing the glow plug.
At low temperatures, since a diesel engine is difficult to start, a glow plug is disposed in a cylinder of the engine and electric current is passed through the glow plug to raise the temperature inside of the cylinder, thus facilitating the starting at low temperatures. Among the glow plugs may be mentioned a sheath heater constituted by a heat resistant metal sheath containing a helical electric heating wire embedded in a powder of heat resistant insulating material, and a heating rod comprising a resistor embedded in a ceramic material.
In a prior art glow plug of the type described above, a heater rod is supported at the fore end of a hollow metal holder and an external connecting terminal is secured to the rear end of the holder. The external connecting terminal and the rear end of the heater rod are interconnected by an electric conductor. With this prior art construction, for the purpose of insulating the external connecting terminal from the holder, an insulating bushing made of resin is inserted through the rear opening of the holder to surround the fore end of the terminal, and the periphery of the rear opening is squeezed to secure the insulating bushing to the holder.
With this construction, the external connecting on terminal is held by the periphery of the holder opening squeezed about the periphery of the insulating bushing to prevent from disengaging in the axial direction. Furthermore, owing to the difference in the thermal expansion coefficients of the insulating bushing and the holder, the bonding therebetween would be loosened.
The insulating bushing is generally made of a thermosetting artificial resin so that when used over a long period, due to a large difference in the ambient temperature, the molded insulating bushing slightly shrinks. This can be readily understood from the fact that the ambient temperature varies greatly during the running and standstill of the diesel engine utilizing the glow plug.
The metal holder is also influenced by temperature variation so that it expands slightly. However, this expansion is not sufficiently large to compensate for the shrinkage of the insulating bushing with the result that a gap is formed between the metal holder and the insulating bushing, thus loosening the external connection terminal.
Where an insulating bushing made of resin is used, and a terminal plate is secured to the external connecting terminal with a screw, it becomes impossible to firmly secure the terminal plate because of the rotation of the external connecting terminal. Rotation of the external connection terminal twists the conductor that interconnects the external terminal and the heater rod, resulting in breakage of the electrical connection. Furthermore, a gap formed between the insulating bushing and the holder permits water or oil to enter into a space in the holder in which the conductor is contained, which causes corrosion of the electric connection.